Enable helper-rust by default also on PHP 8.4

[cataphract]

Summary

• Lowers the threshold for enabling the Rust helper by default from PHP 8.5 to PHP 8.4 (DD_APPSEC_HELPER_RUST_REDIRECTION_DEFAULT)
• Updates integration test build logic to treat PHP 8.4 the same as 8.5 (build helper-rust dependency automatically)
• Removes test8.4-release-zts from the helper-rust disabled CI matrix, since it's now enabled by default on 8.4
• Updates comments throughout to say "PHP 8.4+" instead of "PHP 8.5+"

Test plan

• CI passes for PHP 8.4 with helper-rust enabled by default
• Existing PHP <8.4 tests continue using the C helper by default

[chatgpt-codex-connector]

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[datadog-prod-us1-3]

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 60.64% (-0.05%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 87adbc2 | Docs | Datadog PR Page | Give us feedback!}

[pr-commenter]

Benchmarks [ appsec ]

Benchmark execution time: 2026-04-28 16:14:02

Comparing candidate commit 87adbc2 in PR branch glopes/helper-rust-8.4 with baseline commit 557740f in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 12 metrics, 0 unstable metrics.

[pr-commenter]

Benchmarks [ tracer ]

Benchmark execution time: 2026-04-28 16:53:31

Comparing candidate commit 87adbc2 in PR branch glopes/helper-rust-8.4 with baseline commit 557740f in branch master.

Found 1 performance improvements and 6 performance regressions! Performance is the same for 187 metrics, 0 unstable metrics.

scenario:MessagePackSerializationBench/benchMessagePackSerialization-opcache

• 🟩 execution_time [-6.945µs; -5.375µs] or [-6.395%; -4.948%]

scenario:SamplingRuleMatchingBench/benchRegexMatching1

• 🟥 execution_time [+37.736ns; +115.064ns] or [+2.531%; +7.717%]

scenario:SamplingRuleMatchingBench/benchRegexMatching2

• 🟥 execution_time [+54.249ns; +132.351ns] or [+3.662%; +8.934%]

scenario:SamplingRuleMatchingBench/benchRegexMatching3

• 🟥 execution_time [+31.748ns; +135.452ns] or [+2.138%; +9.123%]

scenario:SamplingRuleMatchingBench/benchRegexMatching4

• 🟥 execution_time [+49.784ns; +114.816ns] or [+3.373%; +7.779%]

scenario:SamplingRuleMatchingBench/benchRegexMatching4-opcache

• 🟥 execution_time [+235.421ns; +422.179ns] or [+2.008%; +3.602%]

scenario:SpanBench/benchOpenTelemetryAPI

• 🟥 execution_time [+15.745µs; +30.981µs] or [+2.193%; +4.314%]